An Introduction to Supernova Remnants What is a Supernova Remnant? A supernova remnant is simply what is left over (i.e. the remnant) of a supernova. That probably isn't a very helpful definition, so I'll try to explain what I mean. First, I should probably define what is meant by the term "supernova." A supernova is simply the explosion of a star. Keep in mind, the average star is a million times more massive than the entire Earth, so we're talking about an incredible explosion here. The energy released in a typical supernova is on the order of 1044 joules. (A joule is simply a unit of energy that physicists use). 1044 is a 1 followed by 44 zeroes! There is no comparison for this amount of energy that is even meaningful. Supernovae are so powerful that they can be seen from halfway across the universe (billions of light years!). Here we can draw a comparison. The farthest star you can see with your eye on a dark night is only a few thousand light-years away. I'll need to talk about what causes a supernova, because it is somewhat relevant here.

Stars live out their lives by burning (via nuclear fusion reactions) light elements like hydrogen into heavier elements like helium in their core. For a star like the sun, this process will go on for about 10 billion years before it runs out of fuel. More massive stars have more fuel to burn, but they go through it much more rapidly, so they actually live shorter lives. When a star runs out of hydrogen, it will try to burn helium into even heavier elements, like carbon, nitrogen, and oxygen. If those elements sound familiar, they should. You're a carbon-based lifeform, and you're breathing nitrogen and oxygen as we speak. All of those materials came from the core of some ancient star that exploded and spread its materials around the galaxy, before the Sun and the Earth were even formed! The cartoon on the right shows a deuterium nucleus combining with a tritium nucleus to for an alpha particle (and a stray neutron). Alpha particles are the nuclei of helium atoms, and can be combined with other particles to form all the elements that we commonly see around us. Supernova Types When a star burns all of its available fuel, one of a few things can happen.

For a star like our sun, which is only an average sized star, death will be a relatively calm affair. In about 5 billion years, the sun will exhaust its fuel supply, and will puff up into a bloated star called a red giant (swallowing up Earth in the process). It will then shed it outer layers off into space, forming a beautiful phenomena known as a planetary nebula. At the center of the nebula will be the remains of what was once our sun, a cinder of a star known as a white-dwarf (white because it will be hot, dwarf because it will be very small, about the size of Earth). Planetary nebulae have nothing to do with planets, the name is something of a misnomer. To the left is an example of a planetary nebulae.

This one is NGC 6543, which is more commonly known as the "Cat's Eye." These nebulae are the remains of stars that were like our sun that have recently "burned-out." Left by themselves, the planetary nebula will fizzle out after a few million years and the white-dwarf star will continue to smolder for billions of years, before eventually cooling off and being nothing more than a cold hunk of dead star material. However, there is something more exciting that can happen. Many stars in the universe are in binary systems, which is a system of two stars that mutually orbit each other. If one of these stars turns into a white dwarf, it can actually suck material off of the other star, provided that it is close enough. If a white dwarf gets too massive, it becomes unstable, and can explode if pushed over a certain limit. The resulting explosion is known as a type Ia supernova. The details of the explosion are quite complicated, and are still being worked out by smart people with fancy computers! To the right is an artist's conception of what a binary system containing a white dwarf and a red giant would look like.

The white dwarf on the right is pulling material off of the big star on the left, and the material is spiraling in to accrete onto the white dwarf. When the white dwarf pulls off enough matter and reaches a critical mass, it will explode! For stars that are much larger and more massive than the Sun, an entirely different scenario unfolds at the end of their life. First of all, as I mentioned above, massive stars actually live shorter lives than smaller stars. So a star that is 10 or so times more massive than the Sun will live for only a few hundred million years. After it has run through all the fusion reactions it can, it starts to collapse in on itself. It does this because there are no more reactions going on to provide pressure outward to counteract the force of gravity, which is always pulling it inward. With one final spectacular burst the outer parts of the star bounce off of the core and the resulting shockwave explodes the star, leaving only the core of the star behind, as either a neutron star or a black hole. These are commonly known as core-collapse supernovae, and include type II, type Ib and type Ic supernovae.

Both types of supernovae are extraordinarily violent events, releasing more energy in a short amount of time than an entire galaxy (hundreds of billions of stars). You might be wondering if there is any danger to Earth, since this sounds like it could be the plot of a bad Hollywood doomsday movie. Not to worry. A supernova would need to be within a few dozen parsecs of Earth to be dangerous to human life. At this time, there are no stars in that immediate vicinity that are candidates to go supernova anytime soon. There are stars relatively close by in our galaxy that could explode within the next few million years (Eta Carina and Betelgeuse, to name two), but we should be far enough away that we will be protected. That's not a guarantee, but an educated guess. Supernovae happen at the rate of about once every fifty to one hundred years for a given galaxy. Teams of astronomers using telescopes all over the world scan the skies every night looking for new supernovae.

Up until a few years ago, it was relatively rare to find one, but thanks to advances in telescope and computer technology, as well as an increase in the number of people working on it, they are becoming commonplace as billions of galaxies are part of nightly automated surveys. We actually detect supernovae at the rate of more than one per night now! Most of these are far, far away, and require advanced telescopes to see them, but occasionally we'll get one in our own neighborhood. The image on the left is an example of what a distant supernova looks like. The fuzzy green part is the galaxy that the star was located in, and the bright spot is supernova 2006gy, a supernova discovered in 2006. The thing that everyone in the field is waiting for, though, is the next supernova in our own galaxy. We have no idea when it will happen. We haven't observed one in the Milky Way in over 300 years. If a star should meet its fate in our local section of the galaxy, the show will be spectacular.

If it happens within a few thousand light-years of Earth, it will easily be the brightest object in the night sky, perhaps even bright enough to look like a star during the day! It will slowly fade over the next 6 months to a year, at which point it will only be visible through a telescope. What happens next leads us to our discussion of what a supernova remnant is. Supernova Remnants What happens after a star goes supernova? An explosion of that magnitude doesn't simply dissipate in a short amount of time (at least not by human standards). A supernova remnant is simply the expanding blast wave from the explosion plowing through outer space, as well as the remains of what was once the star following behind it. The image to the right is an example of a supernova remnant. It is Cassiopeia A, the last supernova in our galaxy. It is the remnant of a supernova that exploded in the late 17th-century, as seen from Earth. The colors in the image represent different things. The faint blue ring around the outside is the outer edge of the blast wave running out into space at a speed of about 6000 kilometers per second.

It is analogous to the ripple wave created when a rock is dropped into a pond. It shows emission from gas located in the interstellar medium (a fancy term for space) that has been heated up to several million degrees when the blast wave slammed into it. The multiple colors on the inside (green, yellow, orange) show emission from material that was once contained in the star. This is called the ejecta, since it was ejected from the star when it exploded. The fuzzy red color represents emission from interstellar dust that has encountered the blast wave and has been heated up. This image combines images from the three Great Observatories that NASA currently has in operation in outer space: The Hubble Space Telescope (HST), the Chandra X-ray Observatory (CXO), and the Spitzer Space Telescope (SST). These telescopes all see the universe in different wavelengths of light, and all provide valuble pieces of information about what is going on when we examine a supernova remnant. For more examples (and pictures!) of supernova remnants that come directly from my personal work, see my page on LMC Supernova Remnants . So why do we study supernova remnants?

What can possibly be learned by studying the remains of an explosion that has already happened? We are primarily interested with the interaction of the shock wave and the ejecta with the interstellar medium, and how the medium evolves as a result of this interaction. The universe is nearly 14 billion years old, and at this point, every point in a galaxy has most likely been overrun by several supernova blast waves and has been mixed with ejecta from multiple stars. By studying present supernova remnants, we can understand how our local region of space got to be like it is. As I've mentioned above, all "heavy" elements in the universe are made either in stars or in supernova explosions. "Heavy," when referred to by astronomers, means anything heavier than hydrogen or helium. The interaction of the shock wave with interstellar dust is also of interest. Dust is important in many areas of astrophysics.It is required to form planetary systems and star-forming regions, and its attenuation and extinction effects on starlight can be measured, giving us an idea of how much dust is in the universe. Studying supernova remnants in the infrared (which is how dust grains radiate) can also give us an idea of the amount of dust present.

Recent work done by myself and my collaborators has shown a discrepancy between these two methods of determining dust content in the ISM. We don't know exactly why this is yet, but we're working on figuring it out. Perhaps it involves some new physics that we haven't considered yet, perhaps it is just a consequence of using rather indirect methods to come up with a measurement. Chandra :: Field Guide to X-ray Sources :: Supernovas & Supernova Remnants Supernovas & Supernova Remnants Quicktime Movie Every 50 years or so, a massive star in our galaxy blows itself apart in a supernova explosion. Supernovas are one of the most violent events in the universe, and the force of the explosion generates a blinding flash of radiation, as well as shock waves analogous to sonic booms. Supernovas were originally classified on the basis of their optical properties. Type II supernovas show conspicuous evidence for hydrogen in the expanding debris ejected in the explosion; Type Ia explosions do not. Recent research has led to a refinement of these types, and a classification in terms of the types of stars that give rise to supernovas.

A Type II, as well as Type Ib and Type Ic explosion, is produced by the catastrophic collapse of the core of a massive star. A Type Ia supernova is produced by a sudden thermonuclear explosion that disintegrates a white dwarf star. Chandra image of SN 1006 Type II supernovas occur in regions with lots of bright, young stars, such as the spiral arms of galaxies. They apparently do not occur in elliptical galaxies, which are dominated by old, low-mass stars. Since bright young stars are typically stars with masses greater than about 10 times the mass of the sun, this and other evidence led to the conclusion that Type II supernovae are produced by massive stars. Some Type I supernovas show many of the characteristics of Type II supernovas. These supernovas, called Type Ib and Type Ic, apparently differ from Type II because they lost their outer hydrogen envelope prior to the explosion. The hydrogen envelope could have been lost by a vigorous outflow of matter prior to the explosion, or because it was pulled away by a companion star.

Core-collapse Supernovas The general picture for Type II, Type Ib and Type Ic supernovas - also called core-collapse supernovas - goes something like this. When the nuclear power source at the center or core of a star is exhausted, the core collapses. In less than a second, a neutron star (or a black hole , if the star is extremely massive) is formed. The formation of a neutron star releases an enormous amount of energy in the form of neutrinos and heat, which reverses the implosion. All but the central neutron star is blown away at speeds in excess of 50 million kilometers per hour as a thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of several billion Suns. Thermonuclear Supernovas Type Ia supernovas, in contrast, are observed in all kinds of galaxies, and are produced by white dwarf stars, the condensed remnant of what used to be sun-like stars.

A white dwarf star, a dense ball primarily composed of carbon and oxygen atoms, is intrinsically the most stable of stars, as long as its mass remains below the so-called Chandrasekhar limit of 1.4 solar masses. Illustration: NASA/CXC/M.Weiss Chandra image of Kepler's Supernova Remnant If, however, accretion of matter from a companion star or the merger with another white dwarf, push a white dwarf star over the Chandrasekhar limit of 1.4 solar masses, the temperature in the core of the white dwarf will rise, triggering explosive nuclear fusion reactions that release an enormous amount of energy. The star explodes in about ten seconds, leaving no remnant. The expanding cloud of ejecta glows brightly for many weeks as radioactive nickel produced in the explosion decays into cobalt and then iron. Supernova 2001el (Lawrence Berkeley National Lab) Because Type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses, they produce about the same amount of light.

This property makes them extremely useful as a distance indicator - if one Type Ia supernova is dimmer than another one, it must be further away by an amount that can be calculated. In recent years Type Ia supernova have been used in this way to determine the rate of expansion of the universe. This research has led to the astounding discovery that the expansion of the universe is accelerating, possibly because the universe is filled with a mysterious substance called dark energy. Pair-Instability Supernovas Artist's Concept: A close-up of SN 2006gy For extremely massive stars, another, even more violent type of supernova is possible. According to stellar evolution theory, temperatures rise to several billion degrees in the central regions of stars with masses between 140 and 260 suns. At these temperatures the usual process of converting mass into energy (E = mc2) by nuclear reactions is reversed, and energy is converted into mass in the form of pairs of electrons and antielectrons, or positrons. The production of electron-positron pairs saps energy from the core of the star, disturbing the equilibrium between the outward push of pressure and the inward crush of gravity.

This so-called "pair instability" causes violent pulsations that eject a large fraction of the outer layers of the star, and eventually disrupt the star completely in a thermonuclear explosion. Pair-instability supernovas, if they exist, would be the most energetic thermonuclear explosions in the universe. In stars with masses greater than about 260 suns, the pulsations would be overwhelmed by gravity and the star would collapse to form a black hole without an explosion. Illustration of Shock Waves in Supernova Remnants For stars with initial masses above about 200 suns, pair-instability supernovas would produce an abundance of radioactive nickel. Radioactive decay of this large mass of nickel into cobalt and other nuclei would feed energy into the expanding debris for several months, and create an ultra-bright supernova. Observations with Chandra and optical telescopes indicate that Supernova 2006gy , the most luminous supernova ever recorded, may be a long-sought (40 years) pair-instability supernova. The intense radiation emitted by a supernova lasts from several months to a few years before fading away.

In the meantime, the rapidly expanding (millions of miles per hour) matter from the explosion eventually crashes into circumstellar gas. This collision creates a supernova remnant consisting of hot gas and high-energy particles that glow in radio through X-ray wavelengths for thousands of years. Chandra image of Cassiopeia A The process of forming the remnant is somewhat like an extreme version of sonic booms produced by the supersonic motion of an airplane. Expanding stellar debris creates a shock wave that races ahead of the debris. This forward shock wave produces sudden, large changes in pressure and temperature behind the shock wave. The forward shock wave also accelerates electrons and other charged particles to extremely high energies. Electrons spiraling around the magnetic field behind the shock wave produce radiation over a wide range of wavelengths. Radiation from supernova remnants is especially conspicuous at radio wavelengths, and radio telescopes have traditionally been the primary tools for discovering these objects. Chandra image of Tycho's Supernova Remnant In recent years, supernova remnants have also been discovered with focusing X-ray telescopes. The X-rays are produced by the forward shock wave and by a reverse shock wave that heats the debris, or ejecta, of the exploded star.

The reverse shock is formed as the high pressure gas behind the forward shock wave expands and pushes back on the stellar ejecta. A Chandra observation of the supernova remnant Cassiopeia A (Cas A) clearly shows both the outer shock wave and the debris heated by the reverse shock wave. The study of supernova remnants with radio, infrared, optical and X-ray telescopes enables astronomers to trace the progress of the shock waves and distribution of elements ejected in the explosion. These data are especially significant because supernovas are the primary means for seeding the galaxy with many elements such as carbon, nitrogen, oxygen, silicon and iron that are necessary for planets and life. In core-collapse supernovas a rapidly spinning neutron star, or pulsar, can produce a pulsating source of radiation and a magnetized nebula of high-energy particles that lights up the interior of the expanding shell. The Crab Nebula, a remnant of a supernova observed in 1054 A.D., is the most spectacular example. Chandra's image of the Crab Nebula reveals rings and jets of high-energy particles that appear to have been flung outward over great distances from the neutron star.

The diameter of the inner ring is about 1,000 times the diameter of our solar system. Infographic (PDF) Chandra has been able to detect numerous pulsars and their associated pulsar nebulas. These discoveries are proving to be one of the best ways to identify supernova remnants produced by the core collapse of a massive star, and to distinguish them from remnants produced by the thermonuclear disruption of a white dwarf star (Type Ia supernova). Another method used to determine the origin of a particular remnant is to study the relative amounts of various elements, especially oxygen and iron. Core-collapse supernovas are rich in oxygen, whereas thermonuclear supernovas produce relatively more iron. The remnants of Tycho's and Kepler's supernovas are thought to have been produced by Type Ia supernovas. More Information This supernova exploded not once, but twice | Astronomy.com This supernova exploded not once, but twice This supernova exploded not once, but twice Astronomers find a superluminous supernova that seems to have exploded twice.

By Jordan Rice | Published: Thursday, July 28, 2016 An artist's interpretation of a supernova NASA Most supernovae explode only once, but a newly found superluminous star did something truly bizarre: it exploded twice. A recently discovered class of supernovae called superluminous supernovae (SLSN) are brighter and longer-lasting. Only a dozen or so have been found, and using the Gran Telescopio CANARIAS (GTC), astronomers have found evidence of another, but reveals surprising behavior. It seems to have exploded not only once, but twice. The findings are published in The Astrophysical Journal Letters . There are several different types of supernovae; the most well known type Ia, where a white dwarf star absorbs mass from another star or collides with another white dwarf which causes a thermonuclear explosion as its mass becomes unstable. The other types of supernovae are caused when massive stars use up all their fuel and can no longer fuse atoms together in its core. They then collapse under their own gravity causing a supernova. The research has revealed that the supernova had an initial increase in brightness, declined for a few days, then increased again even stronger than before.

"Superluminous supernovas are up to a hundred times more energetic than type Ia supernovae because they can remain bright for up to six months before fading, rather than just a few weeks," explains Mathew Smith, the lead author of the study and a postdoctoral researcher at the University of Southampton, in a press release . "What we have managed to observe, which is completely new, is that before the major explosion, there is a shorter, less luminous outburst, which we can pick out because it is followed by a dip in the light curve, and which lasts just a few days." This is the first time something like this has ever been observed in a supernova. "From our data, we have tried to determine if this is a characteristic unique to this object, or whether it is a common feature of all superluminous supernovae, but has not been observed before, which is perfectly possible given their unpredictable nature," Smith says. Discovered on December 21, 2014, the object was given the name DES14X3taz by the Dark Energy Survey (DES) which studies the expansion of the universe through dark energy, but occasionally detects supernovae and other transient phenomenon.

As soon as DES14X3taz was found, the GTC made it a priority for two nights of observation on January 26 and February 6, 2015. The GTC is able to devote some of its time to these transient objects as they may offer unrepeatable opportunities. The GTC has a 10.4 meters mirror that, when combined with its powerful spectroscopy instrument, make it the ideal tool to observe SLSNe. Using data from the GTC and other telescopes, Smith and his team were able to reconstruct the brightness of DES14X3taz almost from the moment of its detection. They have determined its absolute brightness with great precision and its distance of 6.4 billion light-years. The researchers concluded, after comparing their observations to many models, that the most plausible explanation is the birth of a magnetar, or a neutron star rotating very rapidly on its axis. The initial peak, decline, then quick rise back to brightness is consistent with that of an emission of a huge bubble of material that cools as it grows. "We think that a very massive star, some 200 times the mass of the Sun, collapses to form a magnetar.

In the process, the first explosion occurs, which expels into space a quantity of matter equivalent to the mass of our sun, and this gives rise to the first peak of the graph,” says Smith to Phys.org. “The second peak occurs when the star collapses to form the magnetar, which is a very dense object rotating rapidly on its axis, and which heats up the matter expelled from the first explosion. This heating is what generates the second peak in the luminosity.” Understanding this process may allow astronomers to “standardize” the model for SLSN as has already happened for type Ia supernovae. Its large luminosity makes these objects ideal candidates for calculating distances on larger scales and with greater accuracy than with current techniques. First is understanding their origin and their nature. Until now, the only other examples of SLSNe have been in small galaxies with low metallicity (small amounts of heavy elements), which is still not well understood. Smith says, “it is a part of the mystery of these objects.” A huge priority to continue studying these objects is to detect more of the SLSNe and observe them from the moment they explode in real time with a telescope comparable in size to the GTC.

Lives and Deaths of Stars Chapter index in this window   Chapter index in separate window This material (including images) is copyrighted! . See my copyright notice for fair use practices. Select the photographs to display the original source in another window. Links to external sites will be displayed in another window. In this next-to-last stage of a star's life, the outer layers are ejected as the core shrinks to its most compact state. A large amount of mass is lost at this stage as the outer layers are returned to the interstellar medium. For the common low-mass stars (those with masses of 0.08 to about 6 or 7 times the mass of the Sun during their main sequence stage), the increased number of photons flowing outward from the star's hot, compressed core will push on the carbon and silicon grains that have formed in the star's cool outer layers to eject the outer layers and form a planetary nebula. The ultraviolet from the hot exposed core, called a white dwarf, causes the gases to fluoresce. Most noticeable is the red emission from the excited hydrogen and nitrogen, the green emission from doubly-ionized oxygen, and the blue emission from excited helium.

Planetary nebulae can be distinguished from H II regions by their compact shape and strong emission lines of doubly-ionized oxygen (that give them their green color), doubly-ionized neon, and singly-ionized helium. (The image of the Ring Nebula on the left is courtesy of Palomar Observatory .) AAO image Planetary nebula get their name because some looked like round, green planets in early telescopes. They are now known to be entirely different than the planets and are about one or more light years across (much larger than our solar system!). Originally, we thought planetary nebulae were simple expanding spherical shells that look like rings on the sky because when you look along the edge of the expanding spherical shell, you look through more material than when you look toward the center of the shell. The round soap bubbles you made as a child (or still do!) look like rings for the same reason. Indeed, many of the planetary nebulae first seen, like the Ring Nebula in Lyra and the Helix Nebula in Aquarius look like rings.

More complete surveys of the planetary nebulae, high-resolution images from the Hubble Space Telescope, and careful analysis of the various parts of the planetary nebulae have shown that planetary nebulae have much more complex structures. Many have bipolar outflows like the Dumbbell Nebula, Hourglass Nebula, and Eskimo Nebula whose different orientations of their poles with our line of sight cause the differences in their appearance as seen from the Earth. These nebula probably have equatorial rings or disks of material ejected during the red giant phase that force the material to flow perpendicular to the rings/disks. The two rings of the Hourglass Nebula (see the picture below) are centered along the star's poles that are oriented around 60� to our line of sight. The upper ring is around the pole that is coming towards us and the lower ring is around the pole that is oriented away from us.

There is evidence that the Ring Nebula in Lyra is actually bipolar in shape with a torus or doughnut shape around the equator and a roughly cylindrical (hotdog shape) outflow along the poles and we are viewing it from right along one of the poles, so it looks like a simple ring (see the link on Hubblesite.org for a video describing its complex shape ). Also, the Helix Nebula is probably two disks oriented perpendicular to each other. Dumbbell image --- Cat's Eye image Companion stars may also be affecting the shapes and may explain why some, like Hourglass Nebula, have the central white dwarf not centered. Complex ones like the Cat's Eye Nebula seem to show that the star ejected its layers in a series of spherical pulses separated by about 1500 years. There are also jets of high-speed gas and shockwaves of gases of different speeds running into each other. While we have just some rough ideas of the causes of their shapes, we certainly can marvel at their beauty! Hourglass image -- Eskimo image High-resolution images of planetary nebulae show complex structures in the expanding nebula. The picture below is a detailed view of the Helix Nebula from the Hubble Space Telescope .

The expanding gas from the planetary nebula gas ejection runs into gas and dust dispersed in the red giant winds. As it passes the slower moving red giant wind material, the gas shapes the denser blobs into comet-like shapes. Although they are called "comet knots", they are not to be confused with real comets in our solar system. Each of these blobs is over twice the size of our entire solar system! Further explanation of the causes of the sometimes bizarre shapes of the planetary nebula is available at Bruce Balick's homepage. The rare high-mass stars (those with masses of about 8 to 50 times the Sun's mass during their main sequence stage) will go the explosive supernova route. When a massive star's iron core implodes, the protons and electrons fuse together to form neutrons and neutrinos. The core, once the size of the Earth, becomes a very stiff neutron star about the size of a small town in less than a second. The infalling outer layers hit the core and heat up to billions of degrees from the impact. Enough of the huge number of neutrinos produced when the core collapses interact with the gas in outer layers, helping to heat it up.

During the supernova outburst, elements heavier than iron are produced as free neutrons produced in the explosion rapidly combine with heavy nuclei to produce heavier and very rare nuclei like gold, platinum, uranium among others. This happens in about the first 15 minutes of the supernova. The most massive stars may also produce very powerful bursts of gamma-rays that stream out in jets at the poles of the stars at the moment their cores collapse to form a black hole (source of the long gamma-ray bursts---we see only the jets pointed towards us). The superheated gas is blasted into space carrying a lot of the heavy elements produced in the stellar nucleosynthesis process. This explosion is a supernova. As the expanding gas crashes into the surrounding interstellar gas at thousands of kilometers/second, the shock wave heats up the interstellar gas to very high temperatures and it glows. Strong emission lines of neutral oxygen and ionized sulfur distinguish their spectra from planetary nebulae and H II regions. Also, the ratio of the strengths of the individual doubly-ionized oxygen is that expected from shock-wave heating. Planetary nebulae and H II regions are lit up by the action of ultraviolet light on the gas, while supernova glow from shock-wave heating.

The gas from supernova explosions also has strong radio emission with a non-thermal continuous spectrum that is produced by electrons spiralling around magnetic field lines. Gas from recent explosions (within a few thousand years ago) are visible with X-ray telescopes as well. A famous supernova remnant is the Crab Nebula above. Chinese astronomers recorded the explosion on July 4, 1054 and the Anasazi Indians painted at least one picture of it. The Vela supernova (in the constellation Vela; figure below) occurred long before the Crab Nebula so it is much more spread out. Different parts of the expanding gas have run into regions of the interstellar medium of different densities. For that reason and also because there is turbulence in the expanding supernova gas, the remnant seen today is wispy strands of glowing gas. The neutrinos formed when the neutron core is created fly away from the stiff core, carrying most of the energy (over 99%) from the core collapse away with them. Some energy (less than1%) goes into driving the gas envelope outward.

The rest of the energy (less than only 0.01%) goes into making the supernova as bright as 1011 Suns (as bright as an entire galaxy)! When a supernova occurred in a satellite galaxy of the Milky Way at the beginning of 1987 (called SN1987a), the Kakiomande neutrino detector in Japan detected a huge burst of neutrinos from the direction of the satellite galaxy. This provided confirmation of the supernova models. The images below show the star before it went supernova (right frame and arrow) and after the explosion (left frame) Recent views of SN1987a from the Hubble Space Telescope (below) shows the material from the supernova explosion itself expanding outward at over 9.5 million kilometers per hour preferentially into two lobes that are roughly aligned with the bright central ring. The central bright ring and two outer rings are from material ejected by the star before its death. Does this image remind you of the Hourglass Nebula above ? Supernovae are very rare---about one every hundred years in any given galaxy---because the stars that produce them are rare.

However, there are billions of galaxies in the universe, so simple probability says that there should be a few supernovae happening somewhere in the universe during a year and that is what is seen! Because supernovae are so luminous and the energy is concentrated in a small area, they stand out and can be seen from hundreds of millions of light years away. The bright gas nebula of a planetary nebula or supernova does not last long, only a few tens of thousands of years. As the nebula expands, it cools and dims. The processed material becomes part of the interstellar medium in the galaxy. Twenty-five years after supernova 1987A Twenty-five years after supernova 1987A KEITH COOPER ASTRONOMY NOW Posted: 23 February 2012 While primitive humans of the Middle Paleolithic hunted prey and sheltered in caves in Africa, a distant star eighteen times more massive than the Sun, located faraway in the Large Magellanic Cloud (LMC) endured a catastrophic collapse as it reached the end of its life.

As the star caved in, its outer layers rebounded off its dense core and blasted outwards, ripping the star apart in a supernova. Some 160,000 years later the light of this supernova, travelling at 300 million metres per second, finally reached Earth to shine in Southern Hemisphere skies on 24 February 1987. The supernova as it went off in the Tarantula Nebula. Image: ESO. Twenty-five years later supernova (SN) 1987A, as it has become known, is giving astronomers an unprecedented look at what happens to a massive star before and after it explodes. A careful perusal of star charts prior to the supernova allowed the exact star that exploded – Sanduleak (Sk) –69° 202 – to be identified. Sk –69° 202 had been a luminous blue supergiant located on the edge of the great Tarantula Nebula, a giant star-forming region in the LMC.

Here stars are born fast and die hard, the glowing veils of the nebula littered with the whorls of ancient supernova remnants – SN 1987A was merely the latest addition to its collection. The �string of pearls� � clumps of matter in an older ring of debris around SN 1987A that are being heated as shock waves and debris from the supernova crash into them. Image: NSA/P Challis and R Kirshner (Harvard�Smithsonian CfA)/B Sugerman (STScI). The Hubble Space Telescope has been following the progress of the expanding cloud of debris since the early 1990s. It has watched as light emitted from the radioactive decay of elements expelled by the supernova – particularly nickel-56 and cobalt-56 – slowly faded. Meanwhile, shock waves were witnessed beginning to buffet a ring of debris a light year wide that had been expelled by tempestuous convulsions within the star thousands of years before it blew up. Fainter, more distant rings have also been detected by Hubble.

The shock waves have caused clumps of denser gas within the main ring to light up, a ‘string of pearls’ numbering in their dozens progressing in intensity until, in 2011, Hubble observed the debris from SN 1987A reaching a transition point; the fading was reversed as a new power source took over. An artist’s impression of the rings around SN 1987A. Image: ESO/L Calçada. The bulk of the expanding cloud of debris hurled out by the supernova is now smashing into the older ring so violently that it generates X-rays, which excite the gas in the expanding debris and cause it to glow. NASA’s Chandra X-ray Observatory has observed these X-rays, confirming what is happening. SN 1987A is now a true ‘supernova remnant’ in the vein of the Crab Nebula or Cassiopeia A, illuminated not by the explosion of the star but by collisional processes between the debris and material beyond it. Such supernova remnants are highly energised regions: many high-energy cosmic rays are found to be produced by them.

A mystery remains, however; a supernova generally leaves behind what was once the core of the star in the form of a rapidly rotating neutron star: a pulsar. However the regular blip-blip-blip of radio, X-ray or even gamma-rays from a pulsar has yet to be detected. So was a pulsar born in the explosion of Sk –69° 202? Or was the supernova so catastrophic that its core collapsed beyond even the point of a neutron star and instead into a black hole? It is possible that the pulsar is being obscured by dust and, if we are not in the line-of-sight of its radio beams, we may otherwise miss it. What is now known is that there is plenty of dust present to hide a neutron star. A wider view of the region around SN 1987A, showing the inner ring and two outer rings. Image: ESA/Hubble and NASA. Hubble and Chandra are not the only space telescopes to have spied on SN 1987A.

The largest space telescope ever launched, the European Space Agency’s infrared-seeking Herschel Space Observatory, with its 3.5-metre mirror, has also been turned towards the remnant. Whereas Hubble and Chandra are generally observing the warmer, glowing parts of the stellar debris, Herschel sees cold dust at a chilly –250 degrees Celsius in far (long) infrared wavelengths. SN 1987A, it turns out, was like a dust-bomb, with estimates of the total dust it threw into space, based on the infrared brightness of the dust (200 times brighter than the Sun is at the same wavelength of light) implying enough dusty material to build the equivalent of 200,000 Earth-mass planets. Mingled within the dust are elements as diverse as oxygen, nitrogen, sulphur, silicon, carbon and iron. This immense amount of dust has been beyond expectations and, if all supernovae spew out this much dust, it helps explain why young galaxies that we can see existing in the early Universe, which have high rates or star birth and death, are so dusty.

The dust, however, isn’t a nuisance to be wiped away – this is the material that goes into building new planets, moons and even life. The iron in your blood and the calcium in your bones all came from supernovae like SN 1987A, as mostly did the oxygen we breath and the carbon in our constituent molecules (dying Sun-like stars that turn into red giants rather than supernovae also produce lots of dust containing some elements such as oxygen and carbon). The birthplace (or should that be ‘deathbed’?) of SN 1987A: the Tarantula Nebula in the Large Magellanic Cloud. The supernova remnant is too small top be seen, but resides to the middle right of the picture. Image: ESO. SN 1987A was astounding for a variety of reasons. It was the last supernova to be visible to the unaided eye in the night sky (it was first spotted by astronomers Ian Shelton and Oscar Duhalde at the Las Campanas Observatory in Chile). While not a galactic supernova, it was the next best thing, remarkably close in the LMC.

Furthermore, its proximity has allowed us to follow the story of the death of a giant star and, in the form of the outer rings and the identification of the progenitor star in the shape of Sk –69° 202, some of he events leading up to its destruction. Supernovae are popping off all the time in distant galaxies, but nobody knows when the next close, bright supernova will take place. When it does, SN 1987A will have prepared us for what to expect. A torrent of ghost-like neutrino particles had arrived with the light from SN 1987A and its possible our first indication that another nearby supernova has gone off, before it has even been spotted in the skies, will be another burst of neutrinos. Either way, in all likelihood the star has already exploded – we just don’t know it yet. The Planets From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions.

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Question: In astronomy, what name is given to the remnant of a star that once exploded as a supernova?
Answer: Neutron Star